1. Field of the Invention
This invention relates to a device for measuring a light beam by means of a diffraction lattice (or grating). More particularly, the invention is concerned with such a device adaptable to a light metering device in a photographic camera.
2. Description of the Prior Art
The light metering device in a photographic camera splits a portion of an image-forming light beam, which has passed through an object lens 1 with a beam splitter, and the thus split light is detected by a light detector. For the light beam splitter useful for such a light metering device, there has so far been used a single half mirror which is obliquely provided in the camera. Such a beam splitting device for use in the camera, etc., should desirably be as thin as possible. U.S. Pat. No. 3,464,337 discloses a device, in which two members, each being provided on its surface with a plurality of slant reflective surfaces, are cemented together to form a prismatic line raster, and a part of the incident light is split for light metering thereby.
Other prior art patents for light metering devices, include U.S. Pat. Nos. 4,103,153 and 4,178,084. Generally speaking, a light beam, which has passed through an optical system, has a different angle of expansion depending on the F-number of the optical system. Therefore, when various light metering systems having different light metering areas are to be utilized with one light metering device, such metering device should be able to accurately measure a light beam in correspondence to variations in the aperture F-number of the optical system for the incident light beam, irrespective of the light metering system. In other words, the device should have a good proportionality in the F-number without regards to the size of a light splitting area.